Skin Grafting

A wound that will accept a skin graft must be free of infection, free of devitalized tissue, and must have an adequate blood supply. Skin grafts are classified as either split thickness or full thickness and are selected depending on the size of the defect to be covered and the thickness of coverage that is desired for a particular area. Split-thickness skin grafts (STSGs) are typically used to cover defects when cosmesis is not of primary concern and when the size of the defect is too expansive to be covered by a full-thickness skin graft (FTSG). During the harvesting of an STSG, a thin layer of epidermis and dermis is excised using either a dermatome or the Humby knife at a thickness that is predetermined by the adjustable setting that is selected. STSG are often meshed to expand the surface area of the graft, thereby minimizing the size of the necessary donor site while providing drainage holes for blood or serum. FTSGs are usually used to cover a smaller surface area and require primary closure of the donor site. In the harvesting of an FTSG, an ellipse is made around the defect pattern, which is then incised and undercut. The donor site is then closed primarily; or alternatively, if there is too much tension, an STSG may be used to cover the donor site. The effective healing of an STSG depends on the presence of a well-vascularized recipient site, close apposition of the graft to the recipient bed, and appropriate immobilization of the graft to foster development of nascent vascular connections. The 2 basic factors affecting graft take are the ability of the graft to receive nutrients and the presence of vascular ingrowth from the recipient bed. Factors that will negatively impact the healing of a skin graft include patient smoking, radiation, chemotherapeutic or immunomodulating drugs, and malnutrition. Complications of skin grafting include primary and secondary graft contracture, altered function, sensation and hair growth of donor skin, and pigmentation mismatch.


HISTORY
The biological transfer of skin was first reported in 1869 by Jacques Louise Reverdin, a Swiss surgeon working in Paris, who harvested very small slivers of epidermis using a lancet to apply to a forearm wound. 1 In 1870, G.D. Pollack of London reported success in 8 of 16 cases in which he tried small Reverdin grafts. 2 Also in London in 1870, George Lawson presented 3 cases of full-thickness skin grafts (FTSGs) before the Clinical Society of London. 3 L.X.E.L. Ollier of Lyon later described the use of intermediate-thickness skin graft and FTSG in 1872. 4 After the Franco-Prussian War of 1870 to 1871, Carl Thiersch of Leipzig, an accomplished microscopist, did experimental work on skin grafting and recommended very thin split grafts. 5 J.R. Wolfe of Glasgow, in 1875, 6 and Fedor Krause of Germany, in 1893, reported on FTSGs. 7 It was not until 1929 that Vilray P. Blair and James Barrett Brown of St Louis reported on their landmark achievement of using large split-thickness skin grafts (STSGs). 8 This technique was made easier by the invention of the dermatone by Earl Padgett and George Hood in 1939. In Europe at that time, tubed flaps dominated reconstructive surgery. Skin grafting was uncommonly used, except by Sir Archibald McIndoe, who used them for burn injuries. After World War II, however, the dermatome and an improved Blair's knife were introduced, and skin grafting became a common technique for appropriate cases in Europe as well. 2 Since then there have been improvements in graft harvesting techniques and graft stabilization. The underlying science of skin graft healing has also been studied and is now well understood.

WOUNDS REQUIRING SKIN GRAFTS
Simply stated, wounds come in 2 types: those without skin loss and those with skin loss. All wounds must be carefully evaluated, and the treating surgeon must classify the wound by etiology or mechanism, chronology, location, size, and nature of injury. Bleeding must be controlled, infection drained, and necrotic tissue debrided. If a wound can be closed primarily or will heal by secondary intention in a short period of time, then this may be the best option. Larger wounds will require more complicated surgical closure techniques such as skin grafting, local or distant flaps, or both. Generally speaking, skin grafts can only be placed on healthy vascularized tissue. If a wound bed is not adequate for a skin graft, then a flap might be needed. A wound that will accept a skin graft must be free of infection, free of devitalized tissue, and must have an adequate blood supply. In cases where a limb has inadequate arterial inflow, revascularization is necessary. Edematous tissues must be elevated and/or compressed until the edema resolves. Grafts will not adhere to radiated tissues. They usually will not take on bone without periosteum, cartilage without perichondrium, or tendon without paratenon. A skin graft should only be placed on wounds when they are appropriate and ready for grafting.

CLASSIFICATION OF SKIN GRAFTS
Skin grafts are classified as either split thickness or full thickness and are selected depending on the size of the defect to be covered and the thickness of coverage that is desired for a particular area. STSGs include epidermis and varying proportions of the underlying dermis. FTSGs are comprised of the epidermis and the entire thickness of the dermis, including structures such as sweat glands, sebaceous glands, hair follicles, and capillaries. 9

STSGs
STSGs are typically used to cover defects when cosmesis is not of primary concern, and when the size of the defect is too expansive to be covered by an FTSG. STSGs range in thickness from thin (0.008 to 0.012 mm in width), to medium (0.012 to 0.018 mm in width), and thick (0.018 to 0.030 mm in width). The thickness of the graft is dependent upon the thickness of the dermal component; and the thicker the graft, the closer the likelihood that the normal appearance of skin will be retained. Thin STSGs may be harvested from any area in the body and take more readily than FTSGs. 9

FTSGs
FTSGs are usually used to cover a smaller surface area and require primary closure of the donor site. For this reason, and because of a superior cosmetic result, FTSGs are commonly used to cover defects of the face, the hand, or other openly visible areas. The most common donor sites for FTSGs are the ears, upper eyelids, neck, hypothenar eminence, and groin. Because these grafts consist of the entire dermal thickness and include the dermal appendages, FTSGs also maintain the hair pattern of their original location even after transplantation to the recipient site. FTSGs are also less likely to contract over time, making them more suitable than STSGs for covering defects on the hands, extremities, joint extensor surfaces, and face.
The location and quality of the recipient site is of ultimate importance in deciding which type of skin graft to use. A graft has the potential to take on any site that has effective microcirculation, which includes granulation tissue, dermis, fascia, periosteum, perichondrium, paratenon, adipose tissue, and muscle. Bare surfaces of tendon, bone, and cartilage, however, are lacking the microcirculation and collateralization necessary to support the graft. Sometimes, the application of artificial dermis on these surfaces may provide a suitable substrate on which a skin graft may take. 10

OPERATIVE TECHNIQUES STSG
In the harvesting of STSGs, a power-driven dermatome or free-hand dermatome may be used. The free-hand dermatome, although free from an external power source and thus simpler to use, offers less control of the precise depth of the graft. Some centers utilize a local anesthetic pump inserted before graft harvesting into the graft donor site to aid in postoperative pain control. Before the harvesting of the graft, it is helpful to infiltrate the subcutaneous tissue with saline to make the dermis more prominent above the underlying structures. Vaselinebased lubrication also facilitates a friction-free surface between the dermatome and epidermis during harvesting Figures 1-6. 8 During graft harvesting, a thin layer of epidermis and dermis is excised using either a dermatome or the Humby knife at a thickness that is predetermined by the adjustable setting that is selected. When using the dermatome, it is helpful to have an assistant stretch the skin taut on the donor site as the dermatome is engaged and lowered to the skin surface. The dermatome should be turned on while in the air above the donor site and slowly lowered to the donor site at a 45-degree angle. Applying a constant force, the dermatome should be slowly advanced down the length of the donor site until the appropriately sized graft is obtained. At this point, with the power still engaged, the dermatome should be lifted off the skin at a 45-degree angle until it is completely free from the skin surface, at which point the power can be disengaged.
When using a Humby knife to harvest a graft, it should be held with the sharp edge at a 45-degree angle to the skin and manipulated in a back-and-forth motion over tight skin in long, even strokes to separate the graft from the thick dermis. When the desired size graft is taken, the back-and-forth motion is continued while supinating the wrist to remove the knife from the skin. 11 STSGs are often meshed to expand the surface area of the graft, thereby minimizing the size of the necessary donor site while providing drainage holes for blood or serum, which would otherwise collect underneath the graft. Meshing the graft also reduces the rate of disruption by shear forces that would otherwise preclude graft take. In addition, skin graftrecipient sites with surface irregularity are better suited for meshed grafts, as the increased pliability of the graft will allow closer apposition into the contours of an irregular surface.
A mesh expansion ratio of 1.5:1 is typically preferred for most STSGs, although mesh ratios of 2:1 or 3:1 can also be used depending on the defect to be covered.  A skin graft meshing device is used to roll the graft through a set of metal cutters to create a uniform meshed pattern throughout the entirety of the graft. The skin graft is placed on the appropriate side of the carrier and then passed through the machine. Disadvantages of meshing a graft include the characteristic "stocking net" appearance of the resultant graft and the risk of enhancing wound contraction as the small gaps between the skin edges close by contraction. An alternative to using a commercial mesher is using a scalpel to make multiple slices in the graft with the same effect. 12 After meshing, the graft should be placed over a clean, well-vascularized recipient wound bed with the dermal side, which is shinier in appearance, facing downward. Once the graft is appropriately laid over the recipient site, utmost care must be taken to ensure hemostasis and inspection should display no hematoma formation. Saline flushes beneath the graft are useful in evacuating blood clots and therefore allowing better adherence of the graft. The graft should also be arranged to sit directly on the underlying recipient tissue, across all of the topographical variation of the wound. Quilting sutures may be used to anchor the graft onto peaks and valleys of the recipient site where the graft is more likely to shear away from, and thus not take. The edges of the graft are secured with interrupted or running absorbable sutures or skin staples, approximating the graft to the recipient tissue bed without strangulation. Edges of the STSG that overlie intact and healthy epidermis on the recipient bed should be trimmed so that there is minimal overlap. 11 Finally, the STSG is then dressed in the manner according to the surgeon's personal preference. Typically, a layer of nonstick material, such as antibiotic-impregnated gauze, is placed directly over the graft. Alternatively, a layer of antibiotic ointment followed by a dry sterile gauze can be used to help prevent desiccation and infection of the wound. A light pressure dressing providing 10 to 20 mm Hg of pressure with cotton bolster is effective at enhancing graft adherence without causing pressure necrosis. Most recently, negative-pressure wound therapy has proven to be effective at bolstering grafts to recipient beds, thus improving graft outcomes. 11 Recent literature suggests that using a negative-pressure wound therapy device can reduce the time of inpatient wound bed preparation and enhance the rate of graft take due to improved inosculation combined with decreased hematoma and seroma formation, as well as less frictional disruption of the wound bed and skin graft interface. Patients managed with negative-pressure wound therapy are also able to ambulate earlier after surgery due to the stabilizing effect of the negative-pressure wound therapy device, thereby decreasing overall length of inpatient hospitalization. 13 The dressing should be kept in place for 3 to 5 days, unless there is concern for underlying infection that would necessitate earlier evaluation of the wound. With the initial dressing change, utmost care must be taken to prevent shearing

Fortier and Castiglione
Techniques in Orthopaedics$ Volume 27, Number 4, 2012 of the graft away from the underlying recipient bed. Moistening the dressing with saline before its removal will help prevent it from sticking to the underlying graft. The initial dressing change, twice daily wet-to-dry dressing changes or daily application of an antibiotic ointment such as bacitracin or nonstick gauze, will help to protect the graft in its initial healing phases. Some surgeons choose to continue negativepressure wound therapy for the first 7 to 10 days after grafting.
Once the graft appears pink and well adherent to the wound bed, the graft may be exposed to open air with daily application of a gentle moisturizing cream. Both the donor site and graft site should be kept out of direct exposure to the sun, and once fully healed should always be protected with sunscreen.
The STSG harvest site is typically dressed with a semiocclusive dressing that allows the accumulation of sterile fluid under the dressing that will bathe the exposed dermis, thereby initiating reepithelialization. The original operative dressing is typically left intact until reepithelialization is complete. If the fluid collection beneath the semiocclusive dressing appears purulent at any time, it should be removed and appropriate wound care should be initiated. 12 More classically, a fine mesh gauze impregnated with petrolatum ointment or other bacteriostatic substance is placed over the donor site, and the wound is treated with an external heat source to stimulate coagulation and dessication of the exudate within the fine mesh gauze.

FTSG
Because of their superior cosmetic results, FTSGs are typically used in grafting defects on the face, hands, and other openly visible aesthetic surfaces. The posterior surface of the ear extending to the skin overlying the mastoid process is a common donor site for facial grafts due to the proximity of its color and texture when compared with facial skin. Skin of the upper eyelid is another frequently used source of FTSGs for facial skin grafts and can provide considerable coverage in individuals with marked redundancy of the upper eyelid. When requiring larger surface area for coverage, the skin of the lower posterior triangle of the neck in the supraclavicular region provides satisfactory color and texture match for facial skin.
Skin of the thigh or abdomen may be used for larger defects, although these sources tend to have suboptimal color and texture match for facial defects. Thigh and abdominal skin has a thicker dermis than facial skin and is described as providing a "masked" appearance when grafted to the face, although it may be useful in covering defects of the palmar surface of the hand or the sole of the foot. The redundant and mobile skin of the antecubital fossa and groin can be used for repair of a defect on the hand, but these sources are limited by the hair patterns and also by concern for contracture of the donor site over a mobile joint. Small defects of the palmar surface of the hand can be covered by hypothenar eminence grafts.
FTSGs are fitted uniquely to the defect, and the use of a pattern is recommended to ensure that an adequate amount of tissue is excised. An irregular defect can be reconstructed on the donor site by using a marker to designate points mirroring those of the irregular defect before the skin is excised.
Before the excision of the FTSG, infiltration of the subcutaneous tissue with fluid or local anesthetic helps to differentiate the plane between the dermis and subcutaneous adipose tissue. An ellipse is made around the defect pattern, which is then incised and undercut. In the excision of the FTSG, care is taken to leave no adipose tissue on the undersurface of the graft; any adipose tissue that remains adherent to the graft should be excised with scissors. 11 The donor site is then closed primarily; or alternatively, if there is too much tension, an STSG may be used to cover the donor site.

GRAFT HEALING
The effective healing of an STSG depends on the presence of a well-vascularized recipient site, close apposition of the graft to the recipient bed, and appropriate immobilization of the graft to foster development of nascent vascular connections.
The ideal recipient bed is one that granulates rapidly and will therefore take a graft readily. Soft tissues, such as muscle and fascia, generally accept grafts with ease, although the ability of adipose tissue to take a graft is dependent on its vascularity. Connective tissues such as cartilage, bone, and tendon also accept grafts readily if they have an intact vascularized perichondrium, periosteum, or paratenon. Bare cartilage and bone may be able to support a small graft if the tissue is able to provide bridging vessels from the periphery of the recipient bed to support graft take.
The initial stage of graft healing is characterized by adherence of the graft to the recipient bed through a fibrin network, which initiates the generation of vascular buds. This apposition permits plasmatic imbibition, during which the donor tissues receive nourishment by the passive absorption of plasma from the recipient bed through capillary action. This takes place over the first 24 to 48 hours after graft apposition, and during this phase, the graft may appear white and edematous.
The inosculation phase of graft healing begins 48 to 72 hours after grafting and may continue for up to 1 week postoperatively. This phase is characterized by further development of the vascular buds within the fibrin network, which eventually anastomose with preexisting and newly forming blood vessels in the graft-recipient interface. During this phase, the graft may appear mottled or demonstrate variation from an erythematous blush to slight cyanosis. 14 The development of lymphatic flow in the graft begins in parallel with vascularization, and lymphatic flow is typically established by postoperative day 5 or 6. At this point, effective drainage of the excessive fluid that had bathed the graftrecipient interface may occur, resulting in a decreased edema and reduced fluid weight of the graft from days 7 to 9.
The final phase of graft healing is reinnervation of the graft, which typically commences in the first 4 weeks after grafting. Sensation generally returns to the periphery first and then gradually proceeds to the interior of the graft. Full return of sensation is more likely to occur in FTSGs compared with STSGs.
The donor sites of STSGs heal by reepithelialization, during which the epithelial cells from remaining portions of dermal appendages migrate across the newly exposed dermis to establish a new epidermis. Within 3 to 4 weeks of graft harvesting, the epidermis of a healed donor site is fully differentiated. 14

FACTORS AFFECTING GRAFT TAKE
The 2 basic factors affecting graft take are the ability of the graft to receive nutrients and the presence of vascular ingrowth from the recipient bed. Any conditions that preclude the graft from receiving nutrients by decreasing the rate of diffusion will thereby reduce the rate of graft take. Seroma and hematoma formation underneath the graft must be prevented with adequate hemostasis, as well as effective meshing or bolstering of the graft to promote adherence to the recipient bed without underlying collections. Edematous wound beds Techniques in Orthopaedics$ Volume 27, Number 4, 2012 Skin Grafting c 2012 Lippincott Williams & Wilkins provide difficult substrates for grafting, because their recipient beds may prevent adequate diffusion of nutrients from the bed to the graft in the initial stages of graft healing. Ischemic wounds must be evaluated for revascularization before attempting skin grafting.
Vascular ingrowth in a new graft is most commonly disrupted by shear forces between the graft and the recipient bed, thereby decreasing the rate of graft take. Immobilization of the graft is of utmost priority to allow for inosculation of the graft, and this may be achieved by using a bolster dressing or negative-pressure wound therapy. To ensure immobilization of a joint containing a graft, the grafted extremity is often placed in a splint for 3 to 5 days to promote improved graft adherence. 9 The presence of contamination or infection of a wound bed will decrease the rate of graft take due to a higher rate of inflammatory mediators such as nitric oxide, cytokines, and interleukins that stimulate an inflammatory milieu in which the fibrin bonds between the graft and recipient bed are destroyed. Chronic wounds should have a clean, granulating wound bed before attempting grafting. To ensure optimal chance of graft take, preoperative tissue cultures should be obtained in wounds that have a history of infection or in chronic, nonhealing wounds that are likely colonized. Skin grafting is not recommended in wound beds with culture-proven >10 5 organisms/g of tissue. Radiated tissues are not adequately vascularized and thus are not acceptable substrates for STSGs. 15 In wounds where there is only a partial take of a graft, the nonviable tissue should be gently debrided and the remaining defect is left to close by secondary intention or may undergo additional grafting. With either of these approaches, there is an increased risk of scarring, deformity, contracture, and irregularities of the contour of the graft.

FACTORS AFFECTING WOUND HEALING
Adequate wound healing requires effective transport of nutrients and sufficient delivery of oxygen to healing tissues. Oxygen is essential for cellular respiration in the graft bed and for hydroxylation of proline and lysine residues of collagen during wound healing. Although oxygenation in tissue beds can be decreased secondary to impaired delivery (poor cardiac function, anemia, or peripheral vascular disease), oxygenation to a highly metabolic graft bed can also be severely limited by smoking, which acutely stimulates vasoconstriction and delivers toxic compounds into the tissues. Patients undergoing skin grafts should be educated preoperatively of the significant risk of graft failure with concomitant smoking and should be counseled to quit smoking before skin grafting.
Tissues that have undergone radiation therapy display an abnormal healing process that results in excessive synthesis of collagen, thinning of the epidermis, decreased density of blood vessels as well as sebaceous and sweat glands, and pigmentation changes in the dermis. Radiation imposes a higher risk of infection and an overall slower healing process that makes these tissues suboptimal substrates for skin grafting. 15 Malnutrition directly contributes to poor wound healing and should be considered when evaluating a patient for skin grafting. Hypoproteinemia limits the available supply of essential amino acids that is necessary to synthesize collagen and other proteins. Cross-linking of new collagen is essentially impossible without adequate levels of vitamin C, whereas vitamin A is necessary for normal epithelialization and bone function and thus is intimately related to the early phase of graft healing. Zinc deficiency can lead to poor formation of granulation tissue and inhibition of cellular proliferation.
Nutrition laboratories including albumin, prealbumin, and transferrin should be evaluated in preoperative patients where malnutrition could limit their ability to support graft take.
Chemotherapeutic agents are known to impair healing by inhibiting cellular proliferation. Similarly, adrenocortical steroids slow the collagen cross-linking process, thereby weakening incisions and also interfere with wound epithelialization and contraction. The most profound effect of steroids is noted when they are administered several days before or after skin grafting. 11

COMPLICATIONS OF SKIN GRAFTING
One of the most common complications of both STSGs and FTSGs is graft contracture, which can lead to functional limitations and a suboptimal aesthetic result. Primary contraction is a result of the immediate recoil of the elastin fibers in the dermis within a freshly harvested graft. The amount of primary contraction is directly related to the amount of dermis in the graft; thus, FTSGs have more primary contraction when compared with STSGs. FTSGs contract to a surface area as small as 40% of their original surface area, whereas STSGs only contract half as much.
Secondary contracture is caused by contraction of a healed graft due to the action of myofibroblasts in the graft. This is more commonly observed in STSGs compared with FTSGs and contracts to a degree that is inversely proportional to the dermal content of STSGs.
The ability of a graft to carry out functions that it previously preformed in its former location, such as sweating and providing hair growth, is directly related to the thickness of the dermis present and therefore the number of epithelial appendages transferred in the graft. Thinner STSGs tend to be more dry and require regular moisturization in comparison with thicker STSGs. FTSGs have the greatest sensory return in comparison with STSGs due to the transfer of neurilemmal sheaths in the graft, in addition to a greater number of hair follicles. 11 Finally, pigmentation mismatch between the graft and the recipient site is a common complication, especially when attempting to graft defects on the face or hands. Using dermal grafts from the posterior auricular region or eyelid can provide the best aesthetic result for facial defects, although the limited size of these available sources may preclude an ideal pigmentation match in all cases. A general rule is to replace tissue with "like" tissue for optimal pigmentation and texture match. 12